A method of operating a gas combuster/reactor

Abstract: A gas combuster/reactor (50) suitable for use in a Claus sulphur plant (10) is arranged to direct oxygen axially along the reactor and gas substantially across the combuster such that the gas is entrained by the oxygen and swirled around the longitudinal axis of the reactor. This arrangement increases gas/oxygen mixing and keeps the very hot zones created around the oxygen inlet well away from any refractory material (64) used to line the reactor (50).

Description: The present invention relates to a method of operating a gas combuster/reactor. The Claus process is commonly used for recovering sulphur from a feed gas stream rich in hydrogen sulphide. A typical Claus sulphur plant comprises a reaction furnace for partially combusting the feed gas stream with an oxygen containing or oxygen enriched gas, a first means of cooling and condensing sulphur from the combustion effluent, at least one train comprising a reheat means, a catalytic Claus reactor and an additional condensing means for further recovering sulphur from said effluent. Presently known reactors comprise a cylindrical casing into which gas is introduced either axially or tangentially. Air, oxygen enriched air, or oxygen (hereinafter referred to as just oxygen) is introduced into the cylinder generally in the same direction as the gas flow in such a way as to cause mixing of said gas with the air etc prior to combustion thereof. When air and gas are introduced along the axis of the reactor inefficient mixing can result, which causes elements such as ammonia which may be present in the feed gas to be passed therethrough in an unreacted state. If unreacted ammonia is passed to the downstream catalytic stages it can very rapidly form into ammonia salts such as ammonium sulphide, for example, which tends to block and hence reduce the efficiency of the catalytic stage. In order to prevent the pass on of unreacted ammonia from the reactor to the catalytic stages it is necessary to ensure that the temperature of the combustion chamber is sufficiently great as to ensure the ammonia is fully destroyed. In order to ensure complete destruction it is necessary to operate with sufficient residence time and with the reactor temperature at at least 1,200°C and preferably between 1,500°C and 1,650°C. This can be achieved by running the reactor on pure oxygen or oxygen enriched air for example or by splitting the gas flow into two portions and burning the first portion with all of the air required for the Claus reactor and then by introducing the second gas portion of the feed gas into the reactor at a position downstream of this initial combustion zone. This latter method is widely used when more than one feed gas stream is available and where at least one major feed stream contains no ammonia since the second portion of the gas stream must not contain ammonia. It is an object of the present invention to provide a method of operating a reactor suitable for use in the Claus process which reduces and possibly eliminates the carry over of unreacted ammonia to the downstream catalytic stages. Accordingly, the present invention provides a method of operating a gas combuster/reactor comprising a cylinder having a longitudinal axis X, a longitudinally extending wall section and an end portion, said reactor further comprising an oxygen inlet and a gas inlet, characterised by directing an oxygen containing gas axially along a central axis of the interior of said cylinder and directing gas in a direction substantially tangential to the cylinder casing thereby to cause said gas to be swirled around said oxygen containing gas and logitudinal axis (x) and entrained by said oxygen containing gas. By introducing oxygen axially along the cylinder rather than thereacross and the gas tangential to the cylinder rather than axially therealong, mixing and hence combustion is improved as the gas/oxygen mixture spirals around the longitudinal axis. Improved mixing is, to some extent, attributed to the greatly increased gas molecule path length which aids the more complete destruction of materials such as ammonia. An additional advantage of axial gas flow resides in the fact that very hot zones created around the oxygen inlet(s) can be kept well away from the refractory material used to line the interior surface of the cylinder. The hot zones will be directed largely down the axis of the furnace where the available distance between the inlet and outlet to the waste heat boiler is sufficient to allow the temperature to equilibrate to the acceptable overall reactor temperatures. Advantageously, the gas is directed in a direction substantially tangential to and down said cylinder. The method may also include the step of providing a baffle may be provided within said cylinder for redisturbing the flow of gas/oxygen, thereby to cause further mixing thereof and to change the pattern of gas flow to one particularly suited to Claus reaction. Additionally, the method may include the step of providing a second gas inlet downstream of the first gas inlet and introducing further reaction gas into the cylinder. Preferably, the method includes the step of providing the oxygen inlet in the form of a plurality of inlets and providing flow control means for each such inlet and controlling the flow of oxygen into the cylinder. Advantageously, the method may include the step of introducing supplementary air into the cylinder via a separate air injector or via the oxygen inlet or said gas injector. The above mentioned gas burner may form the reaction furnace in an apparatus for the recovery of elemental sulphur from hydrogen sulphide containing gas streams, otherwise and more commonly known as a Claus sulphur plant. The present invention will now be more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a Claus sulphur plant incorporating a burner according to the present invention; Figure 2 is a cross-sectional view of the burner shown in Figure 1; and Figure 3 is a view of the oxygen inlets taken in the direction of arrows A-A in Figure 2. Gas streams comprising hydrogen sulphide are typically produced as waste products or by-products in many industrial processes. For example, acid gas streams comprising carbon dioxide and hydrogen sulphide are typically produced during oil refinery operations in which sulphur is removed from crude oil. It is necessary to treat such hydrogen sulphide containing streams before discharging them to the atmosphere so as to reduce or remove altogether their content of sulphur-containing gases. One well known, widely practised process for the treating of gas stream comprising hydrogen sulphide is the Claus process. This process is based on the reaction between hydrogen sulphide and sulphur dioxide to form sulphur vapour and water vapour in accordance with the equation: SO 2 + 2H 2 S = 2H 2 O + 3S Sulphur exists in the vapour phase in a number of different molecular species such as S 2 , S6 and S 8 according to the temperature. The first stage of the Claus process is to burn approximately a third of the hydrogen sulphide in the incoming gas stream 10 in a burner 12 (figure 1 and figure 2) to form sulphur dioxide and water vapour in accordance with the equation: 2H 2 S + 3O 2 = 2H 2 + 2SO 2 This combustion reaction takes place in a suitable furnace or reactor 14 and normally air is used as a source of oxygen for the purposes of combustion. Reaction between the sulphur dioxide and hydrogen sulphide starts in the combustion zone 16 and then continues downstream of the combustion zone 16. It is, however, a feature of the Claus reaction that at the temperature that is created by the combustion of hydrogen sulphide, it is not possible (with air) to convert more than about 75% of the remaining hydrogen sulphide to sulphur by reaction with sulphur dioxide, and typically between 50 to 70% of the hydrogen sulphide is so converted. It is, however, possible to achieve a higher total conversion in the presence of a catalyst at a reaction temperature in the order of 200 to 450°C by reacting the remaining hydrogen sulphide and sulphur dioxide. Accordingly, after the gases pass out of the furnace they are cooled in a cooler 18 to a temperature at which the sulphur that is formed in the furnace condenses. The sulphur is thus recovered. The gases are then reheated in heater 20 to a temperature suitable for the performance of a catalysed reaction in catalyst 22 between hydrogen sulphide and sulphur dioxide, such temperature typically being in the order of 200°C. Typically, two or three stages of catalytic conversion are performed, with the hydrogen sulphide containing gas stream being reheated immediately upstream of each stage and resulting sulphur being separated from the gas stream by condensation immediately downstream of each stage. The resulting gas mixture now containing only a relatively low concentration of sulphur-containing gases is then typically passed to a tail gas clean-up process or is incinerated 24. Suitable tail gas clean-up processes include the Scot, Beavon and Stretford processes. The present invention, shown in detail in Figure 2, provides a reactor or furnace 50 suitable for use as a combined mixing, burning and reaction stage in a Claus process. The reactor itself comprises a cylinder in the form of cylindrical casing 52 having a longitudinal axis X, a longitudinally extending wall section 54 and an end portion 56. The burner further comprises an oxygen inlet or plurality of inlets 58 for receiving and directing air, oxygen enriched air or oxygen (for the purposes of brevity hereinafter referred to simply as oxygen) axially along the interior 60 of said reactor 50 and a gas inlet 62 or a plurality thereof for directing gas to be entrained by the oxygen and swirled around said longitudinal axis in the direction of axis S. By introducing oxygen axially along the cylinder 52 rather than thereacross and the gas tangentially to the cylinder 52 rather than axially therealong, mixing and hence combustion is improved as the gas/oxygen mixture spirals around the longitudinal axis X. Improved mixing is, to some extent, attributed to the greatly increased gas molecule path length which aids a more complete destruction of materials such as ammonia. An additional advantage of axial oxygen flow resides in the fact that very hot zones created around the oxygen inlet can be kept well away from any refractory material 64 used to line the interior surface of the cylinder. The hot zones will be directed largely down the axis of the furnace 50. Clearly, the higher the combustion temperatures that can be provided the greater the efficiency of oxidising or decomposing components such as ammonia. One or more baffles 66 may be provided within the reactor 50 and downstream of the gas inlet so as to cause further mixing of the gas/oxygen mixture. If desired, several gas inlets 62 may be provided one or more of which may be positioned downstream of the baffle or baffles 66 if fitted. In the present invention the gas inlet or inlets 62 are positioned for directing gas in a direction substantially tangential to the cylindrical casing 52 (as shown in Figure 3) thereby to cause said gas to mix with said oxygen in a smoother manner. In order to facilitate the easy control of oxygen entering the reactor 50, the oxygen inlet may comprise a plurality of inlets and flow controlling means in the form of valve or valves 70 may be provided. Oxygen may be supplied via individual pipes to individual oxygen inlets 58 as shown in Figure 2 or it may be supplied via a control pipe and distributed to the individual inlets 58 as shown in Figure 3. Alternatively, additional oxygen may be introduced by way of supplementary air injectors 72 which may comprise an oxygen inlet 58 or a gas inlet 62.

Claims: A method of operating a gas combuster/reactor (50) comprising a cylinder (52) having a longitudinal axis X, a longitudinally extending wall section (54) and an end portion (56), said reactor (50) further comprising an oxygen inlet (58) and a gas inlet (62), characterised by directing an oxygen containing gas axially along a central axis of the interior (60) of said cylinder (52) and directing gas in a direction substantially tangetial to said cylinder casing (52) thereby to cause said gas to be swirled around said oxygen containing gas and longitudinal axis (x) and entrained by said oxygen containing gas. A method of operating a gas combuster/reactor (50) as claimed in claim 1 characterised by directing gas in a direction substantially tangential to and down said cylinder casing (52). A method of operating a gas combuster/reactor (50) as claimed in claim 1 or claim 2 characterised by providing a baffle (66) within said casing (52) for disturbing the flow of gas/oxygen mixture thereby to cause mixing thereof. A method of operating a gas combuster/reactor (50) as claimed in any one of claims 1 to 3 characterised by providing a second gas inlet (62) downstream of said first gas inlet (60) and introducing further gas into said reactor (50) through said second inlet (62). A method of operating a gas combuster/reactor (50) as claimed in any one of the previous claims characterised by providing said oxygen inlet (58) in the form of a plurality of inlets. A method of operating a gas combuster/reactor (50) as claimed in any one of the preceding claims characterised by the provision of flow control means (70) and by the step of controlling the flow of oxygen into said reactor (50). A method of operating a gas combuster/reactor (50) as claimed in any one of the preceding claims characterised by the provision of an air injector (72) and the step of introducing air into said burner through said injector (72) in addition to said oxygen containing gas. A method of operating a gas combuster/reactor (50) as claimed in claim 7 characterised by the step of injecting oxygen through said air injector (72). A method of operating a gas combuster/reactor (50) as claimed in claim 7 characterised by providing said air injector (72) in the form of a gas inlet (62). A method of operating a gas combuster/reactor (50) as claimed in any one of claims 1 to 9 including the step of recovering sulphur from a feed gas stream rich in hydrogen sulphide.

Inventors: WATSON RICHARD WILLIAM, WATSON, RICHARD WILLIAM
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1994
Main classification: C01B17
IPC codes: B01D53, B01J19, F23D14, C01B17, B01F5, B01J12